1. Technical Field of the Invention
This invention relates to an anti-tapping system for cable televisions, etc., and more particularly to an anti-tapping system capable of securely performing protection of tapping in bidirectional cable television systems.
2. Technical Background of the Invention and Problems Therewith
Generally, in a pay broadcast which provides programs to only subscribers, reception terminals are given their own addresses. In order to prevent a nonsubscriber from tapping a program, only when the coincidence of the addresses of a terminal is detected, the terminal is authorized to view the desired program. That is, the reception terminal having an address corresponding to a polling from the center sends an answer signal to the center, and the center responds to the answer signal to authorize the subscriber to view the desired program and performs processing such as fee-charging.
FIG. 11 is a block diagram showing a conventional anti-tapping system. The center 1 transmits a pay program signal through a trunk 2, and further bidirectional couplers 3 and 4 to the respective subscriber terminals 10 and 20. As an example, subscriber terminal 10 will be taken. A program signal transmitted downstream from the center is separated by a bidirectional coupler 11 and applied through a signal path 12 to a converter tuner 13 where the carrier wave of the received program is converted to a predetermined frequency and then applied to a TV receiver 14.
The converter tuner 13 performs the frequency conversion for only the subscriber concerned. At this time, whether the frequency conversion is permitted or not is controlled due to the fact that a data processor 15 transmits data to the center and receives it therefrom via a modem 16.
Usually, in the case of a pay broadcasting system, the center 1 loads a contracted channel map indicative of a contracted channel corresponding to the address of that subscriber down on each terminal. Each subscriber terminal makes reference to the contracted channel using its own address to control its converter tuner 13 for authorization operation thereof.
Namely, the contracted channel map loaded down by the center 1 using the address obtained by an address generator 17 of the terminal 10 is referenced. Unless the subscriber terminal 10 is authorized by the map thus referenced, the frequency conversion at the converter tuner 13 is stopped, or descrambling operation is stopped to prevent viewing of a pay program.
Charging for each subscriber's viewing of a pay program is processed at the center 1 by forming charging maps in RAMs of the data processors 15 in the respective subscriber equipment to collect them by polling.
As just described above, each contracted channel map showing the relationship between subscriber's address and contracted channel is stored and held by data processor 15 through a modem 16 using a downstream 18. The center 1 collects by polling the charging maps formed in the data processors 15 of the respective subscriber equipment through upstreams 19 to process them as the pay program viewing data.
The above conventional anti-tapping system prevents tapping using the respective terminal addresses alone, so that if a noncontractor steals the subscriber's address and causes an address generator 17 to produce that address, the following illegality will be performed.
It is assumed that the subscriber terminal 10 is a legal subscriber, and that a person of nonsubscriber terminal 20 uses the address of the subscriber terminal 10 by stealth. At the illegal nonsubscriber terminal 20, the converter tuner 13 is then authorized to view a pay program, so that the illegal nonsubscriber 20 can view the pay program illegally.
If the upstream 19 is shut down at the illegal nonsubscriber terminal 20, the transmission of the charging data in response to the polling by the center 1 is stopped and illegality is also performed in the charging operation.
Such illegal use of the subscriber's address is easily possible even if the address generator 17 is of a mechanical type such as a dip switch or of the type which is written into a ROM, so that the normal use of the system is hindered.